Non-invasive techniques for measuring various blood parameters, such as blood oxygen saturation and the concentration of substances contained in the blood (hemoglobin, glucose and other substances) have become very popular, since they do not require the withdrawal of a blood sample from a patient's body. Optical monitoring techniques of the kind specified typically utilize the detection of light transmitted or reflected from the location on the patient's body under measurement, and are based on spectrophotometric measurements enabling the indication of the presence of various blood constituents based on known spectral behaviors of these constituents. Most of these techniques utilize a measurement optical device or probe, designed in a manner to be attached to the patient's finger, which includes an optical assembly for irradiating the finger with light and detecting its light response.
U.S. Pat. No. 5,810,723 discloses an apparatus for the non-invasive monitoring of a patient's carboxyhemoglobin level. The patient breathes oxygen to saturate his blood hemoglobin prior to detection. The apparatus utilizes a clamp with arms holding the patient's finger: one arm supports a light emitting source and the other supports a detector. A microprocessor controls the measurements and processes the detected signals.
U.S. Pat. No. 5,638,816 and its continuation, U.S. Pat. No. 5,860,919, disclose an apparatus for the non-invasive monitoring of blood parameters by applying pressure to the patient's finger, thus inducing an active pulse therein. The induced change of blood volume enables a better signal-to-noise ratio to be obtained.
U.S. Pat. No. 5,782,757 discloses a measuring devices in the form of disposable, folded adhesive sensors with optics embedded therein. The probe is designed so as to fit comfortably onto a patient's fingertip.
All the conventional devices of the kind specified are aimed at measuring enhanced optical pulsatile signals caused by the changes in the volume of the blood containing medium (finger). It is known that a regular optical pulsatile signal is typically 2-3% of the total transmission. The above devices are capable of obtaining the enhanced pulsatile signal that reach 8-10% of the total light transmission intensity. This enhancement of the natural pulsatile signal is a boundary of all conventional techniques of the kind specified.
A different technique is disclosed in a PCT application, International Publication No. WO 99/65384, assigned to the assignee of the present application. This is an occlusion based technique, where the measured signals are not pulsatile. According to this technique, the state of blood cessation is created in a medium under measurement, and measurements are taken during this state. This enables to obtain a significantly enhanced light response of the medium, as compared to that of the previously described techniques dealing with the pulsatile signals. To create such a state of blood cessation, over-systolic pressure needs to be applied to the patient's finger at a location upstream of the area under measurement, with respect to the direction of normal blood flow. Once the blood flow cessation state is established, the optical characteristics start to change dramatically, such that they differ from those of the fleshy medium with a normal blood flow by about 25 to 45%, and sometimes even by 60%. At least two timely separated measurement sessions are performed, each including at least two measurements with different wavelengths of incident radiation. None of the conventional probes is suitable for these purposes. A probe in the form of a finger holder, suitable for applying over systolic pressure to a first location on the patient's finger and applying optical measurements to a second location downstream of the first location, is disclosed in a co-pending U.S. application Ser. No. 09/407,390, assigned to the assignee of the present application.